The polysaccharides are the most abundant carbohydrates in nature and serve a variety of functions, such as energy storage or as components of plant cell walls. Polysaccharides are very large polymers composed of tens to thousands of monosaccharides joined together by glycosidic linkages. The three most abundant polysaccharides are starch, glycogen, and cellulose. These three are referred to as homopolymers because each yields only one type of monosaccharide (glucose) after complete hydrolysis. Heteropolymers may contain sugar acids, amino sugars, or noncarbohydrate substances in addition to monosaccharides. Heteropolymers are common in nature (gums, pectins, and other substances) but will not be discussed further in this textbook. The polysaccharides are nonreducing carbohydrates, are not sweet tasting, and do not undergo mutarotation.


Starch

We consume more than half of our daily carbohydrates from starch.Plants store it in the form of granules, which are particularly abundant in seeds (especially the cereal grains) and tubers, where they serve as a means to store carbohydrates.During reduced photosynthesis periods, starch is broken down into glucose which nourishes the plant.Although potatoes are often considered starchy foods, other plants contain a much greater percentage of starch (potatoes 15%, wheat 55%, corn 65%, and rice 75%).The most common type of starch is white powder.

The polymers in starch are amylose and amylopectin.Approximately 10%–30% of amylose and 70%–90% of amylopectin are found in natural starches.Its linear structure is the same as maltose and is characterized by the same 1,4-glycosidic linkages we found in maltose (PageIndex1 part(a)).Upon experimentation, it was shown that amylose is not a simple chain of glucose units but rather coils up like a spring, with six monomers of glucose per turn (part (b) of Figure (PageIndex{1})).This coil of amylose has just enough room inside its core to accommodate an iodine molecule.As a result of the formation of the amylose-iodine complex, starch takes on a blue-violet color when treated with iodine.Even tiny amounts of starch can be detected using this color test.

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.Amylopectin molecules may contain thousands of glucose units separated by branch points found approximately every 25–30 units (Image (PageIndex{2})).As a result of the branching of the chain, the helical structure of amylopectin is disrupted, so instead of the deep blue-violet color amylose gives when dyed with iodine, amylopectin results in a less intense reddish brown.

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Dextrins are glucose polysaccharides with an intermediate texture.Starch imparts the shine and stiffness to clothing due to the presence of dextrins formed when clothing is ironed.Deftrins are used as adhesives on stamps, envelopes, and labels, as binding agents to hold pills together, and as pastes due to their characteristic stickiness when wet.Because dextrins are more easily digestible than starch, they are frequently used in infant foods.

The complete hydrolysis of starch yields, in successive stages, glucose:

starch → dextrins → maltose → glucose

In the human body, several enzymes known collectively as amylases degrade starch sequentially into usable glucose units.


Glycogen

Glycogen is the body's energy reserve carbohydrate.The liver (4%–8% by weight of tissue) and skeletal muscle cells (0.5–1.0%) are two tissues that store carbohydrates in the form of glycogen.Grains of glycogen are located in the liver and muscle cells, much like starch in plants.To maintain metabolic balance, fasting animals utilize these glycogen reserves during their first day without food.

Glycogen is structurally quite similar to amylopectin, although glycogen is more highly branched (8–12 glucose units between branches) and the branches are shorter. When treated with iodine, glycogen gives a reddish brown color. Glycogen can be broken down into its D-glucose subunits by acid hydrolysis or by the same enzymes that catalyze the breakdown of starch. In animals, the enzyme phosphorylase catalyzes the breakdown of glycogen to phosphate esters of glucose.

In the body, about 70% of all glycogen is stored in muscle cells.Although the liver contains a higher percentage of glycogen (by weight), skeletal muscle, which is much larger, stores a greater amount of glycogen in general.